The present invention relates to hydrogen powered vehicles, such as airplane, and more particularly to improved hydrogen fuel tanks for such vehicles.
Stratospheric vehicles, such as airplanes, have been proposed in recent years as platforms for communication repeaters which provide a variety of communication services. One type of vehicle which can be used for these platforms is a hydrogen powered airplane. The high density of hydrogen can achieve the long endurance at high altitudes needed for commercial communication systems. Compared to gasoline, hydrogen, when combined with atmospheric oxygen, yields about three times the energy density of gasoline. The time aloft duration of airplanes fueled by hydrogen can be measured in weeks.
However, the use of hydrogen has disadvantages which make it difficult to realize the endurance advantage. Hydrogen, even in liquid form, requires a relatively large volume storage tank. Also, the low liquid temperatures required for hydrogen fuel systems necessitate a fuel tank that is well insulated. Due to these properties of hydrogen, conventional designs result in heavy tanks with the weight offsetting most of the endurance improvements that the energy density of hydrogen might provide. These tanks typically include metal shells of sufficient thickness to withstand the internal pressures and to be stable against buckling, and a surrounding solid or powdered insulation layer of a thickness adequate to control the heat flow into the tank from the surrounding environment. For a required storage time of several weeks, such a design results in a very heavy tank on the order of 100% of the weight of the fuel.
Thus, a need exists for a light weight cryogenic fuel tank designed to store hydrogen fuel in an airplane for communication repeater platforms.
The above-stated need is satisfied by the present invention which includes a thin-walled spherical metal shell surrounded by a concentric metal and composite shell separated by a radial insulating gap. The inner shell carries the hydrogen mass and the gap between the shells is evacuated to a high vacuum. The outer shell preferably has a sandwich structure whose skins can be joined with a low conductive material. The mutually facing surfaces of the inner and outer shells are coated with a low emissivity metal, such as copper. The two shells are joined at two opposing equatorial locations. The vacuum gap and low emittance surface finishes on the two shells provide appropriate thermal insulation. External stiffening ribs could also be employed for shell stabilization where desired. An electrical heater conducts a controlled amount of heat to the outer shell. Radiation couples the heat to the inner shell and thus to the fuel in order to control the evaporation rate of the hydrogen. A second electrical heater is placed on the outer skin of the outer shell to prevent icing during ascent and descent.
Other benefits, features, and advantages of the present invention will become apparent from the following description of the invention when viewed in accordance with the accompanying drawings and appended claims.